Closed-loop stepping motor control has the attractive properties of guaranteed step integrity and optimal performance for varying load conditions, but this form of control has failed to attain widespread popularity because existing methods of rotor position detection based on optical, inductive or capacitive effects or the Hall effect are expensive or unreliable. The same problem has also occurred in the development of switched reluctance motors where, in traction applications for example, an optical position encoder may be operating in a particularly harsh environment.
In an attempt to eliminate the optical encoder a so-called "waveform detection" of rotor position has been suggested. With this technique the modulating effect of the motional emf on the current waveform is measured and, since the instantaneous motional emf is dependent on rotor position, it is possible to deduce the rotor position. Waveform detection has the advantage of being low cost but is unreliable in most situations because, for example, it is unable to operate at low speeds where the motional emf component approaches zero. Furthermore it appears that waveform detection can only be applied to the series resistance/constant voltage drives, which are commonly used for small motors of less than 1 kW output.
Most large stepping and switched motors are driven by chopper-type circuits. Such circuits incorporate means for repeatedly chopping the electrical connection from the supply terminals to an excited phase winding when current flow has risen to a predetermined value, which will normally be higher than the rated current of the phase winding. The current flowing in the now disconnected phase winding decays through a by-pass circuit and the connection to the supply terminals is restored after a delay time, which may be the time for the current to decay to a predetermined lower value, normally lower than the rated current of the winding. Such a control may be termed a "bang-bang" current control and it functions to maintain the average current in an excited phase winding near its rated level. Because the average current is normally constant techniques of waveform detection for deducing the position of the rotor are inapplicable.